The present invention relates to a process for continuously manufacturing composites of polymer and cellulosic fibres and to the compounded material obtained therewith. It also relates to an extruder to be used in this process.
It is known to produce fibre reinforced plastics. For instance GB 1,151,964 describes a method for obtaining a plastics material reinforced with brittle fibres such as glass fibres. According to this method a brittle fibre substance is supplied as a continuous strand to the other components of the mixture in such a manner that the fibre breaks to a predetermined length. The apparatus used for this process comprises several mixing and kneading members which are not specified.
Lately, the focus on reinforcing fibres is shifting from glass fibres to certain kinds of cellulosic fibres which have outstanding intrinsic mechanical properties. These have the potential to compete with glass fibres as reinforcing agents in plastics. The specific strength of these agrofibres is 50 to 80 percent of glass fibres, whereas the specific modulus can exceed that of glass fibres. Supplementary benefits include low cost, low density, renewability and (bio)degradability. In addition, they are less abrasive during processing with thermoplastics and do not expose operators to potential safety or health problems.
A major disadvantage of cellulosic fibres is the limited temperature at which they should be processed without losing their additional mechanical properties. Further, it has been proven difficult to obtain a homogeneous mixture of polymer and fibre. This is mainly due to the non-polar polymeric surface versus the highly polar fibre surface which prevents satisfactory fibre/polymer intertwining.
EP 426,619 describes a method of producing panels from a thermoplastic polymer and a thermosensitive filler by means of an extruder having at least three feedingly effective helical extrusion sections and at least two non-feeding kneading sections. The extruder thus comprises at least two kneading zones and at least three extrusion zones. The filler is preferably fed into the second extrusion zone.
When cellulosic fibres are used it is important that during the extrusion process these fibers obtain and maintain a high aspect ratio so as to obtain a compounded material with mechanical properties comparable with those of materials containing glass fibres. This means that the diameter should be as small as possible, preferably so called elementary fibres are used. Further the length of the fibres should be as large as possible.
Obtaining such a high aspect ratio of the fibres in the final product has up to now not been achieved. The problem with extrusion processes according to the state of the art is that the high shear forces in the extruder often result not only in a smaller diameter of the fibres but also in a smaller length of the fibres. This reduces the strength properties of the compounded material considerably, relative to the glass fibre reinforced materials.
Accordingly, a substantial need exists for a continuous process for the production of polymer/cellulosic fibre composites with substantially improved mechanical properties, i.e. its rigidity and its strength. In addition, these resulting property improvements should be valid for multiple fibre sources at broad polymer processing temperature ranges and independent from the polymeric melt behaviour.
The above objects are achieved by the present invention. The present invention relates to a process for continuously manufacturing composites of polymer and cellulosic fibres, comprising the steps of:
a) feeding a polymer upstream into an extruder;
b) melting and mixing the polymer in a zone (A) of the extruder; wherein zone (A) comprises at least one positive transportation screw element,
c) feeding cellulosic fibres into the extruder in a zone (B) of the extruder, which zone (B) is located downstream of zone (A);
d) transporting the mixture of polymer and cellulosic fibres obtaining in zone (B) through a degassing zone (C), which zone (C) is located downstream of zone (B), wherein zone (C) comprises at least one positive transportation screw element and
e) transporting the mixture obtained in zone (C) through a pressure building zone (D) of the extruder, which zone (D) is located downstream of zone (C), wherein zone (D) comprises at least one positive transportation screw element,
f) releasing the mixture obtained in zone (D) into a die,
characterized in that zone (B) comprises at least one positive transportation screw element, at least one kneading section and at least one negative transportation screw element such that in zone (B) of the extruder the cellulosic fibres are fibrillated to obtain cellulosic fibres with an aspect ratio as high as possible, while simultaneously mixing the cellulosic fibres with the melted polymer.
The design of this process is such that during the continuous mixing the cellulosic fibres are opened up to elementary fibres (fibrillation) with a high aspect ratio, which are homogeneously distributed in the polymeric melt. The process results in a compounded material with improved rigidity and strength.
The present invention also relates to an extruder which can be used to carry out the process. The present invention comprises all extruders with two separate feeding ports and a degassing port. The preferred extruder for performing the process of the present invention is a corotating twin-screw extruder. An example of such an extruder is a Berstorff ZE corotating twin-screw extruder with a length to diameter ratio varying from 35 to 40.
As indicated above, the extruder comprises four zones: a zone (A) where a polymer fed to the extruder is melted and mixed; a zone (B) where cellulosic fibres are fed to the extruder, fibrillated to elementary fibres and simultaneously mixed with the polymer; a zone (C) wherein the mixture of polymer and cellulosic fibres obtained in zone (B) is degassed and a zone (D) where pressure is built up.
According to the invention zone (A), which is the polymer melting and mixing zone, comprises at least one positive transportation screw element. Preferably zone (A) further comprises at least one kneading section and at least one negative transportation screw element. Preferably, zone (A) begins at a distance of 20xc3x97D calculated from the beginning of the die, wherein D stands for the diameter of the extrusion screw. Generally zone (A) ends at a distance of 38xc3x97D.
In this application the location of the zones is calculated from the beginning of the die, i.e. from the end of the extruder. This is contrary to general practice in which these distances are calculated from the beginning of the extruder. This is done because according to the invention it is important that the feeding port for the fibres is located as close as possible to the end of the extruder.
Zone (A) can further be divided into four temperature zones. Zone (A1) defines the feeding of the polymer. In this zone, which is generally located between 34xc3x97D and 38xc3x97D, the polymeric material is fed to the extruder. A conventional feeder in combination with a hopper are generally used for this purpose. After being fed to the extruder, the material is transported to zone (A2).
In zone (A2), which is generally located between 30xc3x97D and 34xc3x97D, the polymeric material starts to melt, predominantly by shear forces. From (A2) the material is transported to (A3), which is located between 24xc3x97D and 30xc3x97D. From this zone the polymer is transported to zone (A4) located between 24xc3x97D and 20xc3x97D. Both zone (A3) and (A4) serve to further melt and mix the polymer.
Zone (B), which is the fibre fibrillation and mixing zone, comprises at least one positive transportation screw element, at least one kneading section, preferably at least two kneading sections, and at least one negative transportation screw element, Zone (B) is preferably located at a distance between 8xc3x97D and 20xc3x97D. The at least one kneading section of zone (B) is preferably located at a distance between 10xc3x97D and 13xc3x97D.
Zone (B) comprises two temperature zones (B1) and (B2). In zone (B1) the predried cellulosic fibres are continuously and gravimetrically fed in a conventional manner, for instance from a feeder to a hopper and further to the extruder. The position of this zone is between 14xc3x97D and 20xc3x97D. Preferably the fibres are fed at 16xc3x97D. The amount of fibres fed is such that the weight ratio of the fibres in the final compounded material is control led to a certain value. In addition the fibres start to be distributed in this zone.
In zone (B2) the fibres are fibrillated to elementary fibres. In addition the fibres are distributed homogeneously through the polymer matrix. If more than one positive transportation screw element is present in zone (B2), these elements preferably have a decreasing pitch in the flow direction of the polymer and fibre mixture. This zone is located between 8xc3x97D and 14xc3x97D.
Zone (C), the degassing zone, comprises at least one positive transportation screw element. In this zone water and other thermally unstable components are removed from the compounded mixture. For this purpose a conventional degassing port is present which is connected to a vacuum pump. At the end of this zone when the volatile substances have substantially been removed, binding of the fibres and matrix begins.
Zone (D) the pressure building zone, comprises at least one positive transportation screw element. Preferably this zone comprises at least two positive transportation elements. In that case these elements have a decreasing pitch in the direction of the die. This results in an increase in pressure necessary to press the compounded material through the die.
In this zone further homogeneous distribution of the fibres into the matrix and compaction of the compounded material is accomplished resulting in the penetration of polymeric material into the surface pores and microcracks of the cellulosic fibres. During this process both mechanical interlocking and chemical coupling of matrix and fibres are further increased resulting in optimised fibre/matrix interaction. Zone (D) comprises one temperature zone.
The temperatures of the different temperature zones described above depend upon the kind of polymer used. If the polymer is polyethylene, polypropylene or polystyrene, the following temperature profile may be applied.
The thermoplastic polymers that can be applied in the invention comprise xe2x80x9ccommodityxe2x80x9d plastics like low density polyethylene, high density polyethylene, poly(ethylene-copropylene), polypropylene (homopolymer and copolymer) and polystyrene (homopolymer, copolymer and terpolymers). In addition engineering plastics can be applied. Besides the virgin polymeric material, the recycled grades of the above-mentioned plastics are also applicable in the present invention.
A general definition of cellulosic fibres for the purpose of the present invention is xe2x80x98any fibres where the main constituents are of plant tissue and whose main component consist of xcex1-cellulosexe2x80x99. Preferably annual plant fibres or bast fibres, like flax, hemp, jute and kenaf are used. According to a different embodiment paper fibres such as recycled fibres from newspaper are used. It is also possible to use a combination of different types of fibres such as paper fibres and bast fibres. Annual growth plant fibres can, by means of their intrinsic mechanical properties, generally compete with glass fibres.
Generally the raw cellulosic fibre bundles are between one and five millimeters in diameter. After being fibrillated during the compounding process, the cellulosic fibre diameter generally varies from ten to hundred micrometers, whereas the fibre aspect ratio varies from 7 to 100.
The present process is especially suitable for a feed consisting of raw cellulosic fibres which is very economical, However, it is also possible to feed elementary fibres to the extruder. The present process is then advantageous because the elementary cellulosic fibres retain their high aspect ratio.
The amount of fibres fed to the extruders is such that in the final compounded material the fibre content is 5 wt. % to 50 wt. % based on the weight of the compounded material, preferably 30 wt. % to 40 wt. %, most preferably 40 wt. %.
According to the present invention it is preferred to add a coupling agent to the polymeric material. With a coupling agent is meant a polymer which can be mixed with the polymer matrix and which can be chemically coupled onto the fibres. The preferred polymer to coupling agent ratio for a polyolefinic matrix (e.g. polyethylene, polypropylene) is 70 to 6, with a most preferred ratio of 9 to 16, based on weight. For the polystyrene matrix the preferred polymer to coupling agent ratio is 700 to 60, 450 to 550 being most preferred.
The preferred coupling agent for polyethylene is maleic anhydride grafted polyethylene copolymer. The preferred coupling agent for polypropylene is maleic anhydride grafted polypropylene copolymer. The preferred coupling agent for polystyrene is maleic anhydride grated polystyrene copolymer.
The coupling agent is preferably dry mixed with the polymer before being introduced into the extruder. For this purpose any conventional blender may be used. The mixture thus obtained is fed to the feeder as mentioned above. The polymer and coupling agent are homogeneously mixed in zone (A) of the extruder.
Other additives which can be added to the polymer comprise conventional additives such as pigments, antioxidants, fillers and flame retardants. Examples of fillers that may be used are talc, calcium carbonate and carbon black.
The mechanical properties of the compounded material according to the present invention compared with the properties of the polymer matrix are shown below.
The compounded material exiting the die may be granulated before being further processed. These granules can be formed into articles by means of thermoforming processing techniques such as injection molding and compression molding. It is also possible to directly mould the compounded material obtained into sheets, tubes, profiles, etc. The articles obtained from the compounded material may serve to replace wood, plastic and alternatively filled- or reinforced composite alternatives.